The present invention relates to a burner in general.
More particularly, the invention relates to a solid-fuel burner.
Still more particularly, the invention relates to a burner for burning solid fuel in pulverulent form.
Burners of this general type are already known in the art. For example, a burner for a mixture of pulverulent fuel and combustion-supporting air is described in "VGB Kraftwerkstechnik 59", 1979, pages 98 and 99. That device may be provided with a tube for secondary combustion air, or even with still another combustion-air tube which surrounds the secondary-air tube. The central or primary-air tube of the burner houses the igniter lance which may be oil or gas-operated and which is fired up only during the burner start-up (in either the cold or warm start-up mode) or, if necessary, as a combustion-supporting aid during regular burner operation.
The above and other known burners of the type under discussion must necessarily operate along the just indicated lines, since in operation these burners--dependent upon the specific type of pulverulent fuel being used--permit a reduction of the burner capacity only down to at most 40-60% of their rated capacity. Even if an installation has several such burners and some of them are completely shut down, the heating capacity of the installation can generally at best be reduced only to 25-30% of the rated capacity. A reduction of the momentary heat output below this point--desirable as it may be for any of various reasons--is not feasible, for reasons of stability and to assure the necessary uniform combustion-chamber load. What this means, of course, is that the oil or gas-fired combustion lance cannot be shut down at will, so that--even tough the primary fuel is a pulverulent solid fuel--such installations require a substantial amount of oil or gas just to keep the combustion going.
It need not be specially emphasized that any avoidable use of gas or oil is wasteful, both in terms of the overall energy shortage and in terms of cost effectiveness. This is, of course, especially true in a system which is inherently based on the use of pulverulent solid fuel, i.e. where the use of gas or oil is only incidental and the waste occurs because burner (or system) operation cannot be controlled at will. To be able to effect such control in a pulverulent-fuel system, in particular a coal-dust combustion system, the system would have to be capable of permitting a constant increase in total heat output from about 5% after initial ignition, up to about 30-35% of the full rate load, of course taking into account the required uniformity of combustion-chamber loading. After the 30-35% output is reached the burner (or system) could then be switched over to operate at the installed heat capacity.
Attempts have been made in the prior art to arrive at this goal in a round-about way. Thus, German Pat. No. 923,213 suggests the installation of auxiliary burners with small rated capacity in the system, to be used for the ignition and start-up phases only. However, although this solution is theoretically feasible, it requires additional space which is quite simply often not available in the combustion chamber.
Another proposal, made in German Allowed Application No. 2,933,060, is to install within the primary-air tube a small-dimension and small-capacity coal-dust burner, rather than to use a burner which is fired by oil or gas. This would, of course, eliminate the use of oil or gas; however, the dimensions of the primary-air tube cannot be increased at will and such an auxiliary coal-dust burner must itself have a certain size (to achieve the necessary start-up heat rate of about 20-35% of the main-burner rated capacity) which in many instances makes it impossible to accommodate in the primary-air tube.